A blend of a polycarbonate resin and a styrene-containing copolymer is a well-known resin composition with improved processability maintaining the good notched impact strength. This blend resin composition should further have good flame retardancy as well as high mechanical strength because the resin composition are applied to electric or electronic goods, automobile parts, office supplies, etc.
To provide the polycarbonate resin with good flame retardancy, a halogen-containing flame retardant and/or an antimony-containing compound were used. However, the halogen-containing compound is fatally harmful due to the toxic gases generated during combustion, and therefore nowadays the halogen-free resin compositions are used widely.
As a method for conferring flame-retardancy without using a halogen-based flame retardant, a method using a phosphate ester-based flame retardant is commonly used. U.S. Pat. No. 4,692,488 discloses a thermoplastic resin composition comprising a halogen free aromatic polycarbonate resin, a halogen free SAN (styrene-acrylonitrile) copolymer, halogen free phosphorus compounds as flame retardants, a tetrafluoroethylene polymer and a small amount of ABS (acrylonitrile-butadiene-styrene) copolymer.
U.S. Pat. No. 5,061,745 discloses a thermoplastic resin composition comprising an aromatic polycarbonate resin, an ABS graft copolymer, a copolymer, and a monophosphorous ester. However, the resin composition using the monomeric phosphoric acid ester compound as a flame retardant has very poor heat resistance and shows, so called, “juicing phenomenon” which occurs during molding process because the flame retardant form the laminate on the surface of molding product due to the volatility.
It is known that an oligomeric phosphate is used as a flame retardant. Japanese Patent Publication No. 59-202,240 discloses a method of preparing an oligomeric phosphate and the use of the oligomeric phosphate in a polyamide or polycarbonate resin composition.
In addition, U.S. Pat. No. 5,204,394 discloses a resin composition comprising an aromatic polycarbonate resin, a styrene-containing copolymer or a graft copolymer, and oligomeric phosphate as flame retardants. Although the resin composition improves the juicing phenomenon and heat resistance, but is inferior to the resin composition using the monophosphorous esters as flame retardants in flame retardancy. Accordingly, to maintain a good flame retardancy, the resin composition should contain more flame retardants than in the resin composition containing monophosphorous ester as flame retardants. As a result, the resin composition shows poor mechanical properties due to the high content of a flame retardant.
EP 0 640 655 describes a PC/ABS resin composition containing an aromatic polycarbonate, a vinyl copolymer, a graft copolymer, a combination of a monophosphorus compound and an oligomeric phosphorus compound as flame retardants, and a fluorinated polyolefin. In this case, the resin composition shows juicing phenomenon due to the monophosphorous compound, and deterioration of heat stability. Further the flame retardation ability of the resin composition is lowered due to the oligomeric phosphate ester.
The present inventors have developed a flame retardant thermoplastic resin composition that comprises a polycarbonate resin, a rubber modified vinyl-grafted copolymer, a vinyl copolymer, a mixture of oligomeric phosphoric acid ester compounds as a flame retardant and a fluorinated polyolefin resin, which has a good balance of physical properties such as flame retardant, heat resistance, mechanical strength,impact resistance, heat stability, processability and appearance.